1. Field of the Invention
The present invention relates to a circuit to driver a semiconductor laser diode (hereafter denoted as LD), and an optical transmitter implementing the laser and the driver.
2. Related Background Arts
A driver circuit to driver a semiconductor optical modulator has been disclosed in the U.S. Pat. No. 5,546,218B. The driver disclosed therein is to stabilize the optical modulation by adjusting a current flowing in a path bypassing the optical modulator dynamically to maintain the amplitude of the modulation signal. This driver includes an output circuit and a compensation circuit. The output circuit, which provides a driving signal to the optical modulator corresponding to a modulation signal input from the outside of the driver, includes an active layer put between an n-type and p-type semiconductor layers, one of which is common to a layer of the LD. The compensation circuit may adjust resistance of a path bypassing the optical modulator to keep the drive voltage of the driving signal constant.
One type of the driver circuit has been known as a shunt-driver to divide bias current provided to the LD. However, the shunt-driver is hard to increase the amplitude of the driving current, which is equivalent to enhance the trans-conductance of the driver. A larger trans-conductance of the driver makes the pre-amplifier put in the upstream of the driver simple and light to drive the driver. In the shunt-driver, an n-MOS transistor implemented therein is necessary to widen the gate width to enhance the trans-conductance, which inevitably increases input capacitance and degrades the high frequency performance of the driver. A shunt-driver implementing with an npn bipolar transistor instead of the n-MOS transistor deforms the output the output waveform. It is quite hard to escape the shunt-driver from such subjects inherently caused therein.